Re: [RFC PATCH 1/3] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT

Waiman Long
2021-12-23T02:31:36Z

On 12/22/21 06:41, Sebastian Andrzej Siewior wrote:
> The per-CPU counter are modified with the non-atomic modifier. The
> consistency is ensure by disabling interrupts for the update.
> This breaks on PREEMPT_RT because some sections additionally
> acquire a spinlock_t lock (which becomes sleeping and must not be
> acquired with disabled interrupts). Another problem is that
> mem_cgroup_swapout() expects to be invoked with disabled interrupts
> because the caller has to acquire a spinlock_t which is acquired with
> disabled interrupts. Since spinlock_t never disables interrupts on
> PREEMPT_RT the interrupts are never disabled at this point.
>
> The code is never called from in_irq() context on PREEMPT_RT therefore

How do you guarantee that these percpu update functions won't be called
in in_irq() context for PREEMPT_RT? Do you think we should add a
WARN_ON_ONCE(in_irq()) just to be sure?

Cheers,
Longman